46 FOSSIL FISHES OF THE ENGLISH CHALK. 



anterior portion just enters the roof, separating the two parietals (jxi.), which form 

 a remarkably narrow border, and are traversed by a large transverse slime-canal 

 (seen in B. M. no. P. 5491). The frontals (/r.) form nearly the whole of the 

 cranial roof, expanding to their widest point above the back of the eye and 

 gradually tapering forwards. Their middle portion is smooth and depressed into a 

 shallow fossa ; while their lateral half is strongly ornamented Ijy finely tuberculated, 

 radiating ridges, as shown in fig. 1. The long and narrow otic region does not 

 project outwards beyond the widest part of the frontals. A pair of much-elongated, 

 smooth nasal bones (na.) are indicated by remains on the pointed rostrum. 



The eye is only of moderate size, and the sclerotic is ossified (fig. I (t, scL). A 

 narrow ring of circumorbital plates borders the orbit at least behind and below 

 (B. M. no. P. 1810 a), each plate being traversed by the large slime-canal and 

 ornamented with fine tuberculations which are arranged in radiating lines. The 

 greater part of the cheek must have been naked. 



The mandibular suspensorium is vertical, so that the quadrate articulation is 

 beneath the occipital border. The hyomandibular (PL XII, fig. 2, Iim.) is much 

 expanded at its upper end to articulate with the antero-posteriorly elongated 

 pterotic region ; and it seems to taper below wdiere it must have met the sym- 

 plectic. It bears a large laminar vertical crest on the upper part of its outer face, 

 which is crushed backwards upon the preoperculum in the fossils (cr.). The meta- 

 pterygoid (ntjif.) is rather large and thin, almost triangular in shape, and 

 strengthened by a slight ridge which extends upwards and forwards from its 

 postero-inferior angle. The fan-shaped quadrate {'/n.) is much longer than deep, 

 and also rather thin. The ectopterygoid (PI. IX, fig. 10) is a stout elongated bone, 

 thickened for the support of a powerful dentition. It bears a single series of four 

 well-spaced teeth, which decrease in size backwards. Each tooth is hollow and 

 has a large tumid base, firmly fused with the bone ; while the crown is slightly 

 curved backwards and laterally compressed, with its anterior edge and at least 

 part of its posterior edge quite sharp, and the faces ornamented by fine longi- 

 tudinal striations. Each successional tooth arises in a shallow socket immediately 

 in front of or behind the tooth about to be replaced ; so that the actual position of 

 the teeth differs in different specimens, while a nearly replaced tooth and its 

 successor are often seen side by side. The two foremost ectopterygoid teeth 

 exhibit one posterior barlj, and their sharp edge, which does not extend below the 

 barb behind, is very delicately crenulated. The two hinder teeth are comparatively 

 small and very short and stout, without any barb. In the specimen figured, PL IX, 

 fig. 10, the hindermost tooth is accompanied by its adjoining successor. The same 

 fossil shows the anterior limit of the ectopterygoid, where its thickened end is 

 suturally united with the equally stout palatine (pi.). Other specimens indicate 

 that its upper margin is thin where it meets the laminar entopterygoid, which bears 

 a cluster of minute teeth on its oral face (B. M. no. P. 5491). The palatine is a 



